1. Field of the Invention
The present invention relates to coders and/or decoders and, more particularly, to coders and/or decoders providing scalability.
2. Background Information
An area of current technical development relates to the transmission of digitized video frames over a low bandwidth or low bit rate communications medium. In this context, analog video frames are digitized or, alternatively, the digitized video frames are produced directly, such as by using a charge-coupled device (CCD) camera. Typically, to transmit these digitized video frames over a system that is band limited or has a limited bandwidth, video compression techniques are employed.
The field of video compression is well-known and a number of video compression standards have evolved, such as MPEG 1, (ISO/IEC 11172, Information Technology: coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbits/s, Part 1: Systems; Part 2: Video; Part 3: Audio; Part 4: Conformance Testing, 1993, herein incorporated by reference), MPEG2, (ISO/IEC JTC1/SC29/WG11 CD 13818, generic coding of moving pictures and associated audio, November, 1993, herein incorporated by reference), and H.263 (ITU-T SG 15 WP 15/1 Draft Recommendation H.263, video coding for low bit rate communication, Doc. LBC-95-251, October, 1995, herein incorporated by reference), for example.
One property that is desirable to have when employing video compression techniques is xe2x80x9cscalability.xe2x80x9d In this context, a scaleable bit stream refers to a bit stream which, due to resource constraints, such as network bandwidth or processor capability, may be decoded in part, but sufficient to produce useful images. The quality xe2x80x9cscalesxe2x80x9d with the resource limitation or constraint. This property is desirable because without it substantially the same video signal may be transmitted, for example, across many systems of differing bandwidths due at least in part to the diversity of technology available. Further, over time, bandwidth resources available may change as system resource allocations change. Therefore, it may be desirable to either reduce or improve the video quality as these resource allocations change. However, if the compression scheme employed is not scaleable, then for systems having a bandwidth in excess of a predetermined bandwidth, the video frame received will not improve in quality despite an increase in the bandwidth available. Likewise, for systems having a bandwidth below a particular bandwidth, a compressed video frame utilizing that particular bandwidth for transmission may not be transmitted successfully.
Several approaches for providing scaleable video compression have been proposed. For example, one technique referred to as xe2x80x9cLDCTxe2x80x9d or xe2x80x9clayered DCTxe2x80x9d is described in xe2x80x9cA Layered DCT Codec for Internet Video,xe2x80x9d by E. Amir, S. McCanne, and M. Vetterli, published in Video Proc. IEEE International Conference on Image Processing, ICIP ""96, Lausanne, 1996, Volume 1, pp 13-16, herein incorporated by reference. LDCT codes bit planes of transform coefficients in most significant to least significant order to realize a layered structure. Although LDCT provides a type of scalability by effectively partitioning the DCT coefficients according to relative visual effect, it has some disadvantages. One disadvantage is that the level of improvement or refinement provided by employing additional layers is limited by the quantization step employed to quantize the coefficients. As is well-known, quantization is employed in video compression. For the LDCT, because a single quantization step size is employed, employing additional layers does not result in a significant improvement in the quality of the video image received. However, one advantage of this approach is that its computationally complexity is not overwhelming. Another approach referred to as xe2x80x9cSNR Scalabilityxe2x80x9d is described in MPEG-2. See, for example, Techniques and Standards for Image, Video and Audio Coding, by K. R. Rao and J. J. Hwang, available from Prentice Hall, PTR, New Jersey (1996), herein incorporated by reference. Although this approach provides better image quality improvement as xe2x80x9cenhancement layersxe2x80x9d are employed beyond the xe2x80x9cbase layer,xe2x80x9d a disadvantage of this approach is significant computational complexity.
Unfortunately, such computational complexity may overwhelm state-of-the-art processors or microprocessors.
A need therefore exists for a scaleable coder and/or decoder that addresses the foregoing disadvantages.
Briefly, in accordance with one embodiment of the invention, a method of producing quantization error signal samples for a layer of a multi-layer coder applied to successive video frames includes: processing in the transform domain quantization error signal samples produced by the immediately preceding layer. In this embodiment, processing includes using reference quantization error signal samples if the quantization error for the quantization error signal samples exceeds a predetermined criterion or threshold.
Briefly, in accordance with another embodiment of the invention, a method of reconstructing quantized signal samples in the transform domain for a layer of a multi-layer decoder other than a base layer includes: inverse quantizing the signal samples in the transform domain; and summing the inverse quantized signal samples with previous signal samples inverse quantized in the transform domain for this layer if the previous signal samples were subtracted during coding in the transform domain to produce the signal samples.